The present invention is particularly applicable for use in a system to detect the edge of a moving strip, such as a metal strip, as it is being processed and it will be described with particular reference therein; however, the invention has broader applications and may be used in other detectors of the type using a pulsating radiation emitting device, such as a fluorescent tube.
In various manufacturing processes, a long length of strip material, such as metal, fabric, paper, etc. are conveyed from location-to-location along conveying devices. It is generally necessary to correct any variation of the path along which the strip is moving. As the speed of the processing increases, more minute changes in the path cause substantial alignment difficulties. In addition, a rapid corrective measure predicated upon slight variations in the path must be made to assure efficient high speed operation involving the conveying of strip material. To maintain the correct path, several types of systems have been employed to detect the position of the edge of the moving strip. In some systems, both edges of the strip are detected to create a centering system as disclosed in U.S. Pat. No. 4,147,477. One of the more successful systems, whether of the centering type or of the single edge type, is a system employing pulsating radiation having a pulsating frequency which can be recognized by a detector. Such a system is illustrated in U.S. Pat. No. 3,739,177. In this type of system, the frequency of the pulses can be relatively high, in practice, approximately 5.0 K Hertz. In this manner, radiation, such as light, from a radiation emitting device, such as a fluorescent tube, can be pulsated at the known frequency so that the actual light being used for edge or center detection is received by the detector for purposes of correcting any variation in the desired path of the strip being conveyed. These systems are highly successful. The high pulsing frequency precludes interference from low frequency sources, such as ambient light which can include light reflected from rotating components of adjacent machinery and equipment.
Irrespective of the advantages realized by pulsating radiant detectors, these detectors heretofore have had a relatively low ratio of peak light to ambient light, i.e. signal-to-noise ratio. Thus, a brighter and more powerful light was required. In addition, the distance between the emitter and the detector had to be relatively close due to the low signal-to-noise ratio. Also, it was not possible to optimize efficiency of the radiating device since the pulsating light had to be adequate for discrimination. For that reason, the actual efficiency of the pulsating detector system was not high and the heating effect and the lifetime of the radiant tubes was relatively short. Also, the heating due to the necessity for increasing the power to obtain the desired results could cause some difficulty with other components associated with the pulsating detector systems. Since these prior systems used a transformer having a secondary in series with the emitting device, such as a fluorescent tube, there was a substantial tendency to create spikes and interference in the primary circuit due to the negative resistance characteristic of the tube as it fired to initiate each pulse. For that reason, the rate of pulsing of the tube through the transformer had to be maintained at lower levels. This tendency to create feedback interference and spikes in the primary circuit of the transformer was increased due to the fact that the circuit including the fluorescent tube required a current limiter, such as a capacitor or choke. These components have a tendency to store substantial energy which can cause interference and damage to the driving circuit for the transformer as the frequency of operation increases. These prior systems also had little capability of controlling the intensity of the light created by pulsing the radiation emitter or fluorescent tube. Still further, as the prior driving circuit for the transformer fluctuated due to differences in the incoming voltage, there was a corresponding change in the operating characteristics of the secondary circuit of the transformer as it fired the fluorescent tube in both the positive and negative directions. These disadvantages of the prior pulsating system do not affect their advantage over edge detecting or centering systems used prior to these pulsating devices; however, they are disadvantages to which the present invention is directed to make the pulsating radiation edge detector systems highly efficient and more responsive than now possible.